LED is a crystal having p-n junction. When a forward voltage is applied thereto, electron moves from N-region to the p-n junction and electron hole moves from P-region to the p-n junction and light is emitted at the time of recombination of the electron and the electron hole. Therefore since free electrons are in a combined state and at that time, the resulting energy is released in the form of light, LED emits only a light of limited color tones.
Namely, LED having a red LED chip emits only red light, LED having a green LED chip emits only green light, and LED having a blue LED chip emits only blue light.
However a light emitting diode being capable of changing color of emitted light for various applications is strongly demanded.
Usually for changing color of light emission, for example, combination of a red LED chip, green LED chip and blue LED chip are set in a light emitting diode, and a color change is achieved by changing the combination of the respective chips and emitting a light.
In such a light emitting diode, different four terminals are required for the above-mentioned three color LED chips, and therefore there is a problem that a structure design thereof is complicated and further when any one of three color LED chips is broken, a balance of color tone is lost and intended light emission cannot be obtained.
Also there is proposed a white LED lamp produced by providing a layer of fluorescent substance of YAG (yttrium aluminum garnet) on a blue light emitting diode chip.
In this white LED lamp, white light is produced from light excited by the fluorescent substance layer and afterglow of the blue light emitting diode.
However in the above-mentioned white LED lamp, an amount of fluorescent substance on the blue light emitting diode chip is very small and a color tone tends to be changed greatly by an error in the amount of fluorescent substance and by a processing method. Therefore it is difficult to produce a uniform LED and differences in color and brightness arise inevitably, resulting in a low yield and a high cost.
In order to solve those problems, JP11-87784 proposes that a covering material containing a fluorescent substance and a resin acting as a binder thereof is attached to a light emitting diode for adjusting to an intended color tone. Namely, kind and amount of the fluorescent substance to be contained in the covering material are adjusted, and further a coloring agent is contained as case demands and the covering material is formed into a sheet, cap or the like and provided on the light emitting diode, thus producing a desired color tone freely with high reproducibility.
According to the investigation by the present inventors, the covering material disclosed in JP11-87784 is insufficient in a light emitting intensity and light emitting efficiency, and in order to achieve an intended color tone and color emission, it is necessary to increase a content of the fluorescent substance in the covering material and set a thickness of the covering material high.
However even by a trial of increasing a content of the fluorescent substance in the covering material for increasing a light emitting intensity, there is a limit in an amount of fluorescent substance which can be mixed, and if the ratio of the fluorescent substance is increased excessively, dispersion failure and white turbidity arise, and as a result, light transmission is lowered and intended color and light emission cannot be obtained.
The present inventors have made intensive studies with respect to a laminated article which has a photofunctional layer containing a rare earth metal compound being capable of exhibiting a fluorescence (light emission) phenomenon and light amplifying phenomenon, and as a result, have found that a light emitting intensity, light emitting efficiency and further a fluorescence life are enhanced more firstly by using a specific polymer as a matrix polymer (binder resin) of a photofunctional layer and secondly by providing a low refractive index layer on the photofunctional layer and selecting a specific refractive index for the transparent substrate, photofunctional layer and low refractive index layer, and have completed the present invention.